h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I'm a Junior at a US High School [poop] and I'm in one of my 'Let's spend hours look up info on Uni stuff and then get depressed,etc.' moods. So, a few things..

. Is there a Early Decision er thing like we have?
. Is there double majoring?
. I would really like to go to Cambridge (surprise!) but with my grades, is it even okay/healthy to be dreaming/thinking of going there? And what else should I be looking at? The other 2 on my list of possibles are Warwick and KCL.. (nothing below a B [yet], most of my classes are honours/AP; I'm rather stupid and find that my main issue is the stupidest mistakes, which makes me want to cry; my rank is horrible at this inane smart HS of mine: 16% and dropping; blah blah blah)
. So far I'm thinking of doing Philos for Undergrad with something else, and then going into Medicine... would that even be possible if I went to a UK Uni?
. Any advice? On anything?

I don't know if I can help you there, but I am planning to try and go to UC Berkeley... I will be an international student (I live in Ireland) and as far as I can see, at the moment, I don't think my grades are that good.

My American friend has informed me that the SAT tests are incredibily hard and the system is very strict in who they take in.

Understandably, but would you have any idea about what I would have to do and how I would have to do it to try and gain a place there?